Diabetes mellitus is a term generally used to refer to various pathological states characterized by hyperglycemia and altered metabolism of lipids, carbohydrates and proteins. These conditions are also often associated with other co-morbidities, such as obesity and an increased risk of cardiovascular disease. By some estimates, as many as 600,000 new individuals become clinically diabetic every year in the United States.
Diabetic conditions are generally classified as either insulin-dependent diabetes mellitus (IDDM, Type I diabetes) or non-insulin-dependent diabetes mellitus (NIDDM, Type II diabetes). There are also less common clinical pathologies that are associated with diabetic conditions, such as gestational maturity-onset diabetes of youth (MODY), tropical diabetes secondary to chronic pancreatis, diabetes secondary to pancreatic disease or surgery, and diabetes secondary to endocrinopathies.
Virtually all forms of diabetes are due to a decrease in the circulating concentration of insulin (insulin deficiency) and/or a decrease in the response of peripheral tissues to insulin (insulin resistance). These abnormalities lead to alterations in the metabolism of carbohydrates, lipids, ketones and amino acids, and a hyperglycemic condition. IDDM appears to have an autoimmune etiology, which results in destruction of β islet cells in the pancreas and the resulting inability to produce insulin. The etiology of NIDDM, the most prevalent form of diabetes, is more complex and possibly heterogeneous. Some loss of β-cell volume is generally noted in these patients, as well as decreased circulating levels of insulin. NIDDM patients may also suffer commonly from insulin resistance.
The best-established therapy for all IDDM and many NIDDM patients is subcutaneous insulin treatment. Additionally, insulin is used as the treatment of choice for patients with postpancreatectomy diabetes or gestational diabetes. While insulin is a key element in the control of these hyperglycemic conditions, there are a number of limitations associated with its use, including hypoglycemia, allergic reactions to insulin, lipoatrophy, lipohypertrophy, body weight gain, edema, and insulin resistance. There are a number of new forms of insulin on the market or in various stages of clinical evaluation, including new delivery systems, various recombinant forms, new routes of administration, and gene therapy. These novel forms of insulin treatments are believed to share some of the same limitations outlined above. A significant improvement in the treatment of diabetes can be achieved if insulin treatment is combined with agents that increase the insulin sensitivity of the peripheral tissues.
The concept of combination therapy is well exploited in current medical practice. Treatment of a pathology by combining two or more agents that target the same pathogen or biochemical pathway sometimes results in greater efficacy and diminished side effects relative to the use of the therapeutically relevant dose of each agent alone. In some cases, the efficacy of the drug combination is additive (the efficacy of the combination is approximately equal to the sum of the effects of each drug alone), but in other cases the effect can be synergistic (the efficacy of the combination is greater than the sum of the effects of each drug given alone). In real medical practice, it is often quite difficult to determine if drug combinations are additive or synergistic.
For most diabetic patients, treatment involves some form of insulin therapy. In addition, IDDM patients may receive a biguanide (e.g., metformin) to enhance the insulin utilization by peripheral tissues. NIDDM patients are often treated with a combination of insulin, a sulfonylurea (to enhance insulin production in the pancreas) and a biguanide or glitazone (to enhance insulin sensitivity by peripheral tissues). For example, the improved utility of a glitazone in combination with a sulfonylurea was recently demonstrated in human clinical trials (see, WO 98/36755). Recently, two glitazone compounds (rosiglitazone and pioglitazone) were approved in the United States for the treatment of NIDDM patients in combination with metformin.
A variety of antidiabetic compounds are known. For example, sulfonylureas are a group of drugs that induce hypoglycemia by stimulating insulin release from the pancreas. Generally, sulfonylureas have found wide utility in the treatment of NIDDM. Their efficacy is decreased in IDDM because of the inherent inability of the patient to produce insulin. Adverse reactions to sulfonylureas occur in a fraction of patients, particularly the elderly. One of the most severe side effects is hypoglycemia and coma. Other side effects include nausea and vomiting, cholestatic jaundice, agranulocytosis, cardiovascular mortality, aplastic and hemolytic anemias, generalized hypersensitivity reactions and dermatological reactions.
Biguanides are another group of drugs, first introduced in the mid 1950's, that have shown efficacy in the treatment of hyperglycemia by mechanisms that are not well understood. The best known agents of this type include metformin, phenformin and buformin. Unlike the sulfonylureas, metformin does not induce release of insulin from the pancreas. It is thought that its effects are mediated by increasing insulin activity in peripheral tissues, reducing hepatic glucose output due to inhibition of gluconeogenesis and reducing the absorption of glucose from the intestine. Side effects associated with the use of biguanides include lactic acidosis, diarrhea, nausea, and anorexia. These agents are often given in combination with drugs that increase the output of insulin from the pancreas, such as the sulfonylureas, which sometimes results in greater efficacy and/or the ability to use lower doses of the drugs, with an improved side effect profile.
More recently, the glitazones have been introduced and are widely used in the treatment of NIDDM. These agents, also known generically as thiazolidinediones, such as troglitazone, rosiglitazone and pioglitazone, are thought to work by increasing the sensitivity of peripheral tissues, such as skeletal muscle, towards insulin. They are often used in combination with insulin or other agents, such as the sulfonylureas, that enhance the release of insulin from the pancreas. A number of side effects have been described during the clinical evaluation of these agents, including hepatotoxicity, organomegaly, edema, anemia and body weight gain. While hepatotoxicity may be the most acutely life-threatening of these conditions, it does not appear in a large percentage of the patient population. On the other hand, the increases in body weight gain associated with chronic glitazone treatment are generally perceived as worsening an already critical co-morbid condition in the majority of the diabetic patients, and may ultimately result in the loss of antidiabetic efficacy for this type of agent after chronic treatment.
α-Glucosidase inhibitors, such as acarbose, reduce intestinal absorption of starch, dextrin, and disaccharides by inhibiting the action of intestinal brush border α-glucosidase. Inhibition of this enzyme slows the absorption of carbohydrates and the rise in plasma glucose that normally follows after a meal is blunted. Acarbose has shown some benefit in IDDM and NIDDM patients, but is often associated with dose-related malabsorption, flatulence and abdominal bloating.
Other types of agents that have found limited utility in treating diabetes include potassium channel antagonists such as repaglinide, and aldose reductase inhibitors such as zopolrestat and tolrestat. Still in the experimental stage, glucagon antagonists, activators of the retinoid-X receptor (RXR), activators of PPARα, activators of PPARδ and anti-obesity agents are also being evaluated as potential antidiabetic agents.
In view of the foregoing, there remains a need in the art to provide more efficacious treatment for diabetic conditions and diabetic complications. Combination therapy treatments are needed that will reduce the amount of drugs taken, thereby decreasing side effects. The present invention fulfills these and other needs.